The invention relates to a high-strength lightweight steel and its use for car parts and facade linings.
All of the content indications below are in percentage of mass.
High-strength construction steel types have been developed for the vehicle industry with different properties and have already been used in production to a significant extent. A reduction in weight as compared with conventional soft steel can be obtained due to a reduction of sheet thickness thanks to greater strength. To ensure sufficient corrosion resistance, different surface coating processes were developed (Stahl-Eisen-Werkstoffblatt SEW 094 and SEW 093; Stahl und Eisen 106 (1986), No. 12, pages 21-38 and 114 (1994), No. 7, pages 47-53).
Steel types with greater aluminum content are known. Thus, EP-A-0 495 121 discloses steel with up to 7% Al, more than 0.5% Si, 0.1 to 8% Mn and less than 0.01% C, N, O, P for an attenuation of vibration and noise.
EP-A-0 401 098 takes steel types into account with less than 3.3% Si and 1.5 and 8% Al for soft magnetic sheets with a sharp (100) (001) texture (cube layer). Interstitial impurities must be below 50 ppm, C below 30 ppm. The texture used is unsuitable for deformation processes such as deep drawing and stretch forming.
DE 43 03 316 A describes steel types with 13 to 16% Al and in part larger contents in other alloy elements (Cr, Nb, Ta, W, Si, B, Ti) for oxidation and corrosion resistant parts.
DE 32 01 816 A indicates alloys with 1 to 10% Al for parts which come into contact with hydrocarbon-containing liquids at high temperatures (in the range of 750 to 900xc2x0 C.) so that no hydrocarbon deposits occur. The surface of the parts can be pre-oxidized.
The above-described state of the art has the following disadvantages:
Weight reduction can only be achieved by reducing the sheet thickness or through additional constructive and/or joining measures;
The required corrosion protection can only be achieved by providing additional surface coatings.
Deep-drawn steel types containing greater amounts of aluminum which can be well formed or deep-drawn and stretch-drawn, cold rolled and annealed with re-crystallization, such as are needed in the automotive technology or as facade linings, are not part of the state of the art.
It is therefore the object of the present invention to create a steel with a density significantly below 7.6 g/cm3, with greater strength and good cold formability and at the same time better resistance to atmospheric corrosion than conventional deep-drawn steel.
The purely ferritic steel according to the invention is characterized by more than 5 to 9% Al,  less than 0.2% Si, 0.03 to 0.2% Mn, the remainder iron and impurities caused by melting, including up to a maximum of 1% in all of Cu+Mo+W+Co+Cr+Ni and a maximum of 0.1% in all of Sc+Y+rare earths. It may contain in addition
up to 0.1% C
up to 0.5% in all of Ti+Zr+Hf+V+Nb+Ta
up to 0.01% B
up to 0.1% P.
The aluminum content is preferably in a range between 7 and 9%. Furthermore the steel may be alloyed with titanium and/or niobium of at least 0.03%.
The following are special characteristics of the steel composition:
the steel according to the invention is purely ferritic;
it is limited to a maximum of 0.2% content in Si
it contains a small amount of carbon, below 0.1% and no alloy-significant content in Cu, Mo, W, Co, Cr, Ni, Se Y and rare earth metals.
The steel according to the invention possesses an unexpectedly good combination of previously unknown, advantageous characteristics which can be described as follows:
Strength characteristics are markedly higher than those of conventional, soft deep-drawn steel;
Formability, as measured against strength, is comparatively good;
Density is distinctly lower than that of conventional deep-drawn steel types;
Corrosion resistance is considerably improved.
The deep-drawable and stretch-drawable steel with higher content in aluminum is melted down, is poured into a billet, rolled within a temperature range above re-crystallization temperature, or is poured off in form of a band. The steel is either processed directly as a hot band or is cold rolled after hot rolling, with a degree of deformation of more than 20%. The cold band is then re-crystallized and annealed.
Due to its good cold formability and low density, clearly below 7.6 g/cm3, the steel in form of sheets is especially well suited for applications in the automotive industry and as facade linings.